The present invention relates to an optical disc drive for recording data to and/or reproducing data from an optical disc.
Recently, optical discs having a higher recording density have been proposed. One of such optical discs is a DVD (digital versatile disc). The technical standard for the DVD is divided into some subgroups including standards for a DVD-ROM dedicated to reproduction of information, a DVD±R supporting one-time recording, a DVD-RAM supporting multiple recordings, and a DVD-RW. Since, in general, structures for recordation of information vary among the subgroups even if the subgroups belong to a common technical standard, it is required to generate servo signals suitably matching a structure (i.e., a subgroup) of an optical disc being used so that recording and reproducing operation can be properly performed.
Frequently, an astigmatic method is employed in an optical disc drive to generate a focus error signal which is one of the servo signals. According to the astigmatic method, astigmatism is added to light returning from an optical disc, and then the returning light is incident on a sensor having four separate sensor segments. In the sensor, the focus error signal is generated based on a difference between sum of output signals of a pair of diagonally arranged sensor segments and sum of output signals of the other pair of diagonally arranged sensor segments.
If the astigmatic method is used for an optical disc having a land/groove structure on a recording surface thereof, a possibility that a proper focus error signal is not generated arises. That is because there is a possibility that, when a beam spot from which the focus error signal is generated passes the land/groove structure in accordance with tracking movement of an objective lens, the output signals of the sensor segments are badly affected. Consequently, the focus error signal may be badly affected and thereby precise servo control may be disrupted.
Japanese Patent Provisional Publication No. 2005-203043A (hereafter, referred to as JP 2005-203043A) discloses an optical disc drive configured to solve the above mention drawback of the astigmatic method. In the optical disc drive disclosed in JP 2005-203043A, a main beam and two sub-beams are generated for servo control, and the main beam is used for generation of a focus error signal in addition to using the two sub-beams for generation of the focus error signal. In the optical disc drive, the main and sub-beams are arranged such that positions of beam spots of the two sub-beams on the recording surface of the optical disc shift in a radial direction (i.e., to the inside and the outside of the optical disc) and in a circumferential direction of the optical disc with respect to the beam spot of the main beam.
According to the configuration of the optical disc drive disclosed in JP 2005-203043A, phases of fluctuations caused on the two sub-beams when the two sub-beams are reflected by the recording surface have an inverse relationship. Therefore, by combining the signals obtained from the two sub-beams (or by combining the signals from the main and two sub-beams), a suitable focus error signal can be obtained. Frequently, such a technique for generating a suitable focus error signal is called a differential astigmatic method.
In the optical disc drive disclosed in JP 2005-203043A, the interval between a beam spot of the main beam and a beam spot of each sub-beam is extremely small. Therefore, a sensor for receiving the main beam and a sensor for receiving each sub-beam have to be closely located with respect to each other. Therefore, if defocus is caused, a part of a blurred beam spot of the main beam may undesirably enter one of the sensors for the sub-beams. In general, the sensor for the sub-beam is adjusted to have a high gain because the intensity of the sub-beam is smaller than that of the main beam. Therefore, even a part of the blurred main beam may be detected by the sensor for the sub-beam as a relatively large noise.
It is understood that a level of such a noise caused by a part of a blurred main beam entering the sensor for the sub-beam can be decreased by increasing the intensity of the sub-beam. However, to increase the intensity of the sub-beam, it is required to decrease the intensity of the main beam because the main and sub beams are produced in the optical disc drive by splitting a single laser beam emitted from a single light source through use of a diffraction element. Decreasing of the intensity of the main beam may raise other problems such as decrease of a level of an RF signal and deterioration of a property of fast recording.
It is understood that a level of such a noise caused by a part of a blurred main beam entering the sensor for the sub-beam can be decreased by increasing the interval between the sensor for the main beam and the sensor for each sub-beam. However, if the interval between the sensor for the main beam and the sensor for the sub-beam is increased, the interval between the beam spot of the main beam and the beam spot of the sub-beam increases accordingly. In this case, an effective angle of view of an objective lens has to be increased. If the objective lens has a relatively large effective angle of view, unexpected aberrations may be caused. For this reason, increasing of the interval between the beam spot of the main beam and the beam spot of the sub-beam is inappropriate.